Are TrkB receptor agonists the right tool to fulfill the promises for a therapeutic value of the brain-derived neurotrophic factor?

Stress-induced changes in the molecular processes underlying fear memories: implications for PTSD and relevant animal models

Most of the fear literature on humans and animals tests healthy individuals. However, fear memories can differ between healthy individuals and those previously exposed to traumatic stress, such as a car accident, sexual abuse, military combat and personal assault. Traumatic stress can lead to post-traumatic stress disorder (PTSD) which presents alterations in fear memories, such as an impairment of fear extinction and extinction recall. PTSD-like animal models are exposed to a single highly stressful experience in the laboratory, such as stress immobilization or single-prolonged stress. Some days later, animals exposed to a PTSD-like model can be tested in fear procedures that help uncover molecular mechanisms of fear memories. In this review, there are discussed the molecular mechanisms in stress-induced fear memories of patients with PTSD and PTSD-like animal models. The focus is on the effects of estradiol and cortisol/corticosterone hormones and of different genes, such as FKBP prolyl isomerase 5 gene (FKBP5) - FK506 binding protein 51 (FKBP51), pituitary adenylate cyclase-activating peptide (PACAP) - pituitary adenylate cyclase-activating polypeptide type I receptor (PAC1R), endocannabinoid (eCB) system and the tropomyosin receptor kinase B (TrkB) - brain-derived neurotrophic factor (BDNF). The conclusion is that greater emphasis should be placed on investigating the molecular mechanisms of fear memories in PTSD, through direct testing of patients with PTSD or the use of relevant PTSD-like models.

The effects of exercise interventions on brain-derived neurotrophic factor levels in children and adolescents: a meta-analysis

Brain-derived neurotrophic factor is a crucial neurotrophic factor that plays a significant role in brain health. Although the vast majority of meta-analyses have confirmed that exercise interventions can increase brain-derived neurotrophic factor levels in children and adolescents, the effects of specific types of exercise on brain-derived neurotrophic factor levels are still controversial. To address this issue, we used meta-analytic methods to quantitatively evaluate, analyze, and integrate relevant studies. Our goals were to formulate general conclusions regarding the use of exercise interventions, explore the physiological mechanisms by which exercise improves brain health and cognitive ability in children and adolescents, and provide a reliable foundation for follow-up research. We used the PubMed, Web of Science, Science Direct, Springer, Wiley Online Library, Weipu, Wanfang, and China National Knowledge Infrastructure databases to search for randomized controlled trials examining the influences of exercise interventions on brain-derived neurotrophic factor levels in children and adolescents. The extracted data were analyzed using ReviewManager 5.3. According to the inclusion criteria, we assessed randomized controlled trials in which the samples were mainly children and adolescents, and the outcome indicators were measured before and after the intervention. We excluded animal experiments, studies that lacked a control group, and those that did not report quantitative results. The mean difference (MD; before versus after intervention) was used to evaluate the effect of exercise on brain-derived neurotrophic factor levels in children and adolescents. Overall, 531 participants (60 children and 471 adolescents, 10.9-16.1 years) were included from 13 randomized controlled trials. Heterogeneity was evaluated using the Q statistic and I2 test provided by ReviewManager software. The meta-analysis showed that there was no heterogeneity among the studies (P = 0.67, I2 = 0.00%). The combined effect of the interventions was significant (MD = 2.88, 95% CI: 1.53-4.22, P < 0.0001), indicating that the brain-derived neurotrophic factor levels of the children and adolescents in the exercise group were significantly higher than those in the control group. In conclusion, different types of exercise interventions significantly increased brain-derived neurotrophic factor levels in children and adolescents. However, because of the small sample size of this meta-analysis, more high-quality research is needed to verify our conclusions. This meta-analysis was registered at PROSPERO (registration ID: CRD42023439408).

Deciphering genetic causality between plasma BDNF and 91 circulating inflammatory proteins through bidirectional mendelian randomization

Prior studies reported an association between the levels of brain-derived neurotrophic factor (BDNF) circulating in the bloodstream and those of different inflammatory factors. However, their causal relationship remains unclear. Here, we performed a Mendelian randomization (MR) study to investigate the causal relationships between plasma BDNF levels and 91 circulating inflammatory proteins to shed light on the possible role of BDNF in the pathogenesis and progression of inflammation-related neurological diseases in order to distinguish correlation from possible causal effects. Data for plasma BDNF levels were derived from a genome-wide association study (GWAS) encompassing 3,301 European participants. Genetic association estimates for 91 inflammation proteins were extracted from a GWAS meta-analysis that enrolled 14,824 European participants. The primary MR analysis employed the inverse variance weighted (IVW) method and was corroborated by additional methods including MR-Egger, weighted median, weighted mode, and simple mode. Analyses of sensitivity were performed by evaluating the heterogeneity, horizontal pleiotropy, and robustness of the results. Genetic evidence indicated that elevated plasma BDNF levels possibly contribute to decreased concentrations of 13 inflammation proteins (OR: 0.951-0.977), including beta-nerve growth factor (Beta-NGF), caspase 8 (CASP-8), interleukin-15 receptor subunit alpha (IL-15RA), interleukin-17 A (IL-17 A), interleukin-17 C (IL-17 C), interleukin-2 (IL-2), interleukin-20 (IL-20), interleukin-20 receptor subunit alpha (IL-20RA), interleukin-24 (IL-24), interleukin-33 (IL-33), leukemia inhibitory factor (LIF), neurturin (NRTN), as well as neurotrophin-3 (NT-3). The associations between BDNF and IL-33 remained statistically significant after FDR correction (FDR > 0.05). Furthermore, reverse MR analysis showed that C-C motif chemokine 23 (CCL23), CUB domain-containing protein 1 (CDCP1), and NRTN is suggestive for a positive causal effect on BDNF plasma levels (OR: 1.240-1.422). Moreover, 5 proteins are likely to be associated with lower plasma levels of BDNF (OR: 0.742-0.971), including adenosine deaminase (ADA), cystatin D (CST5), interleukin-13 (IL-13), interleukin-17 A (IL-17 A), and vascular endothelial growth factor A (VEGF-A). Genetically determined plasma BDNF levels influence IL-33 and are possibly associated with 12 circulating inflammatory proteins. The data suggest that 8 inflammatory proteins exhibit either negative or protective roles to BDNF levels, respectively. Of these, 5 are negatively associated with BDNF levels, while 3 play protective roles. These findings may offer new theoretical and empirical insights into the pathogenesis and progression of inflammation-related neurological diseases.

Neuroprotective and plasticity promoting effects of repetitive transcranial magnetic stimulation (rTMS): A role for microglia

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation technique used to modulate neocortical excitability, with expanding applications in neurological and psychiatric disorders. However, the cellular and molecular mechanisms underlying its effects, particularly the role of microglia -the resident immune cells of the central nervous system- remain poorly understood. This review synthesizes recent findings on how different rTMS protocols influence microglial function under physiological conditions and in disease models. Emerging evidence indicates that rTMS modulates microglial activation, promoting neuroprotective and plasticity-enhancing processes not only in models of brain disorders, such as Alzheimer's and Parkinson's disease, but also in healthy neural circuits. While much of the current research has focused on the inflammatory profile of microglia, critical aspects such as activity-dependent synaptic remodeling, phagocytic activity, and process motility remain underexplored. Given the substantial heterogeneity of microglial responses across brain regions, age, and sex, as well as their differential roles in health and disease, a deeper understanding of their involvement in rTMS-induced plasticity is essential. Future studies should integrate selective microglial manipulation and advanced structural, functional, and molecular profiling techniques to clarify their causal involvement. Addressing these gaps will be pivotal in optimizing rTMS protocols and maximizing its therapeutic potential across a spectrum of neurological and neuropsychiatric conditions.

Targeting TrkB-PSD-95 coupling to mitigate neurological disorders

Tropomyosin receptor kinase B (TrkB) signaling plays a pivotal role in dendritic growth and dendritic spine formation to promote learning and memory. The activity-dependent release of brain-derived neurotrophic factor at synapses binds to pre- or postsynaptic TrkB resulting in the strengthening of synapses, reflected by long-term potentiation. Postsynaptically, the association of postsynaptic density protein-95 with TrkB enhances phospholipase Cγ-Ca2+/calmodulin-dependent protein kinase II and phosphatidylinositol 3-kinase-mechanistic target of rapamycin signaling required for long-term potentiation. In this review, we discuss TrkB-postsynaptic density protein-95 coupling as a promising strategy to magnify brain-derived neurotrophic factor signaling towards the development of novel therapeutics for specific neurological disorders. A reduction of TrkB signaling has been observed in neurodegenerative disorders, such as Alzheimer's disease and Huntington's disease, and enhancement of postsynaptic density protein-95 association with TrkB signaling could mitigate the observed deficiency of neuronal connectivity in schizophrenia and depression. Treatment with brain-derived neurotrophic factor is problematic, due to poor pharmacokinetics, low brain penetration, and side effects resulting from activation of the p75 neurotrophin receptor or the truncated TrkB.T1 isoform. Although TrkB agonists and antibodies that activate TrkB are being intensively investigated, they cannot distinguish the multiple human TrkB splicing isoforms or cell type-specific functions. Targeting TrkB-postsynaptic density protein-95 coupling provides an alternative approach to specifically boost TrkB signaling at localized synaptic sites versus global stimulation that risks many adverse side effects.

Allium Macrostemon Bge. Attenuates the Cognitive Decline of Aging Mice by Enhancing BDNF/TrkB Pathway

Allium macrostemon Bge. (AM) is a widely utilized culinary spice recognized for its numerous health-promoting properties. Aging-related cognitive impairment (ARCI) represents a significant global health concern during the aging process. However, the potential of AM to attenuate ARCI has not been investigated. This work aims to reveal the effects and potential mechanisms of the water extraction of AM (WEAM) in alleviating ARCI, with a particular emphasis on the BDNF/TrkB signaling pathway. The findings showed a significant enhancement in memory function and a reduction in hippocampal neuronal damage in aging mice following treatment with WEAM, manifested by an increased spontaneous alternation rate in the Y-maze, prolonged step-through latency, and decreased number of errors in the PAT test, a shortened escape latency and increased platform swimming time and platform crossing times in the MWM test. Additionally, WEAM reduced oxidative stress, elevated the expression of proteins related to synaptic plasticity (SYN and PSD95), and activated the BDNF/TrkB signaling pathway in D-galactose-induced aging mice. To elucidate the mechanism by which WEAM alleviates ARCI, both a TrkB activator (7,8-DHF) and an inhibitor (ANA-12) were employed. The results demonstrated that the effects of WEAM on synaptic plasticity were potentiated by 7,8-DHF and diminished by ANA-12. Finally, 11 chemical compositions of WEAM were analyzed and quantified using HPLC-MS/MS, including macrostemonoside, sarsasapogenin, diosgenin, timosaponin AIII, N-p-trans-coumaroyltyramine, guanosine, adenosine, phenylalanine, adenine, arginine, and valine. These results suggest that AM may serve as a promising culinary spice for mitigating ARCI by promoting the BDNF/TrkB signaling pathway, thereby enhancing synaptic plasticity.

Psychedelic-Induced Neural Plasticity: A Comprehensive Review and a Discussion of Clinical Implications

Background: Psychedelics are increasingly recognized as a promising and innovative treatment strategy for several mental disorders. However, there is still a lively controversy in the medical community as regards the rationale of their employment, specifically their indications and potential dangers. Methods: A comprehensive literature search on "MEDLINE/PubMed" and "Web of Science" was performed from inception to 26 June 2024, cross-checking the obtained references. We included all studies, i.e., both clinical and preclinical, that supplied original data. Results: We initially obtained a total of 1083 entries, 813 from MEDLINE/PubMed and 270 from Web of Science. After duplicate elimination, 903 underwent systematic literature selection. Primary abstract screening yielded a total of 572 candidates for eligibility assessment and excluded 331 entries on formal grounds. Eligibility assessment led to the exclusion of 501 titles. Finally, a total of 70 articles were included in this review. Discussion: Preclinical evidence from genetic expression, histology and behavioral studies is soundly consistent with psychedelics possessing neuroplasticity-inducing properties. Despite methodological difficulties, clinical evidence seems to be inferentially in agreement with preclinical findings. However, it is still unclear whether the "neuroplastic boost" induced by classic psychedelics might be dissociable from the psychodysleptic effects, thereby reducing the psychopathological hazards implied by these compounds. Moreover, the fact that the so-called "relaxation of priors" should be unconditionally beneficial appears debatable, and further research should clarify the possible indications and contraindications of psychedelic psychoplastogens within a precision medicine perspective.

Selective, Intrinsically Fluorescent Trk Modulating Probes

Neurotrophins (NTs) elicit the growth, survival, and differentiation of neurons and other neuroectoderm tissues via activation of Trk receptors. Hot spots for NT·Trk interactions involve three neurotrophin loops. Mimicry of these using "cyclo-organopeptides" comprising loop sequences cyclized onto endocyclic organic fragments accounts for a few of the low molecular mass Trk agonists or modulators reported so far; the majority are nonpeptidic small molecules accessed without molecular design and identified in random screens. It has proven difficult to verify activities induced by low molecular mass substances are due to Trk activation (rather than via other receptors), enhanced Trk expression, enhanced NT expression, or other pathways. Consequently, identification of selective probes for the various Trk receptors (e.g., A, B, and C) has been very challenging. Further, a key feature of probes for early stage assays is that they should be easily detectable, and none of the compounds reported to date are. In this work, we designed novel cyclo-organopeptide derivatives where the organic fragment is a BODIPY fluor and found ones that selectively, though not specifically, activate TrkA, B, or C. One of the assays used to reach this conclusion (binding to live Trk-expressing cells) relied on intrinsic fluorescence in the tested materials. Consequently, this work established low molecular mass Trk-selective probes exhibiting neuroprotective effects.

A Combined Computational and Experimental Approach to Studying Tropomyosin Kinase Receptor B Binders for Potential Treatment of Neurodegenerative Diseases

Tropomyosin kinase receptor B (TrkB) has been explored as a therapeutic target for neurological and psychiatric disorders. However, the development of TrkB agonists was hindered by our poor understanding of the TrkB agonist binding location and affinity (both affect the regulation of disorder types). This motivated us to develop a combined computational and experimental approach to study TrkB binders. First, we developed a docking method to simulate the binding affinity of TrkB and binders identified by our magnetic drug screening platform from Gotu kola extracts. The Fred Docking scores from the docking computation showed strong agreement with the experimental results. Subsequently, using this screening platform, we identified a list of compounds from the NIH clinical collection library and applied the same docking studies. From the Fred Docking scores, we selected two compounds for TrkB activation tests. Interestingly, the ability of the compounds to increase dendritic arborization in hippocampal neurons matched well with the computational results. Finally, we performed a detailed binding analysis of the top candidates and compared them with the best-characterized TrkB agonist, 7,8-dyhydroxyflavon. The screening platform directly identifies TrkB binders, and the computational approach allows for the quick selection of top candidates with potential biological activities based on the docking scores.

Positive Allosteric Modulators of Trk Receptors for the Treatment of Alzheimer's Disease

Neurotrophins are important regulators of neuronal and non-neuronal functions. As such, the neurotrophins and their receptors, the tropomyosin receptor kinase (Trk) family of receptor tyrosine kinases, has attracted intense research interest and their role in multiple diseases including Alzheimer's disease has been described. Attempts to administer neurotrophins to patients have been reported, but the clinical trials have so far have been hampered by side effects or a lack of clear efficacy. Thus, much of the focus during recent years has been on identifying small molecules acting as agonists or positive allosteric modulators (PAMs) of Trk receptors. Two examples of successful discovery and development of PAMs are the TrkA-PAM E2511 and the pan-Trk PAM ACD856. E2511 has been reported to have disease-modifying effects in preclinical models, whereas ACD856 demonstrates both a symptomatic and a disease-modifying effect in preclinical models. Both molecules have reached the stage of clinical development and were reported to be safe and well tolerated in clinical phase 1 studies, albeit with different pharmacokinetic profiles. These two emerging small molecules are interesting examples of possible novel symptomatic and disease-modifying treatments that could complement the existing anti-amyloid monoclonal antibodies for the treatment of Alzheimer's disease. This review aims to present the concept of positive allosteric modulators of the Trk receptors as a novel future treatment option for Alzheimer's disease and other neurodegenerative and cognitive disorders, and the current preclinical and clinical data supporting this new concept. Preclinical data indicate dual mechanisms, not only as cognitive enhancers, but also a tentative neurorestorative function.

A TrkB and TrkC partial agonist restores deficits in synaptic function and promotes activity-dependent synaptic and microglial transcriptomic changes in a late-stage Alzheimer's mouse model

INTRODUCTION

Tropomyosin related kinase B (TrkB) and C (TrkC) receptor signaling promotes synaptic plasticity and interacts with pathways affected by amyloid beta (Aβ) toxicity. Upregulating TrkB/C signaling could reduce Alzheimer's disease (AD)-related degenerative signaling, memory loss, and synaptic dysfunction.

METHODS

PTX-BD10-2 (BD10-2), a small molecule TrkB/C receptor partial agonist, was orally administered to aged London/Swedish-APP mutant mice (APPL/S) and wild-type controls. Effects on memory and hippocampal long-term potentiation (LTP) were assessed using electrophysiology, behavioral studies, immunoblotting, immunofluorescence staining, and RNA sequencing.

RESULTS

In APPL/S mice, BD10-2 treatment improved memory and LTP deficits. This was accompanied by normalized phosphorylation of protein kinase B (Akt), calcium-calmodulin-dependent kinase II (CaMKII), and AMPA-type glutamate receptors containing the subunit GluA1; enhanced activity-dependent recruitment of synaptic proteins; and increased excitatory synapse number. BD10-2 also had potentially favorable effects on LTP-dependent complement pathway and synaptic gene transcription.

DISCUSSION

BD10-2 prevented APPL/S/Aβ-associated memory and LTP deficits, reduced abnormalities in synapse-related signaling and activity-dependent transcription of synaptic genes, and bolstered transcriptional changes associated with microglial immune response.

HIGHLIGHTS

Small molecule modulation of tropomyosin related kinase B (TrkB) and C (TrkC) restores long-term potentiation (LTP) and behavior in an Alzheimer's disease (AD) model. Modulation of TrkB and TrkC regulates synaptic activity-dependent transcription. TrkB and TrkC receptors are candidate targets for translational therapeutics. Electrophysiology combined with transcriptomics elucidates synaptic restoration. LTP identifies neuron and microglia AD-relevant human-mouse co-expression modules.

Polyglutamine disorders: Pathogenesis and potential drug interventions

Polyglutamine/poly(Q) diseases are a group nine hereditary neurodegenerative disorders caused due to abnormally expanded stretches of CAG trinucleotide in functionally distinct genes. All human poly(Q) diseases are characterized by the formation of microscopically discernable poly(Q) positive aggregates, the inclusion bodies. These toxic inclusion bodies are responsible for the impairment of several cellular pathways such as autophagy, transcription, cell death, etc., that culminate in disease manifestation. Although, these diseases remain largely without treatment, extensive research has generated mounting evidences that various events of poly(Q) pathogenesis can be developed as potential drug targets. The present review article briefly discusses the key events of disease pathogenesis, model system-based investigations that support the development of effective therapeutic interventions against pathogenesis of human poly(Q) disorders, and a comprehensive list of pharmacological and bioactive compounds that have been experimentally shown to alleviate poly(Q)-mediated neurotoxicity. Interestingly, due to the common cause of pathogenesis, all poly(Q) diseases share etiology, thus, findings from one disease can be potentially extrapolated to other poly(Q) diseases as well.

TrkB transmembrane domain: bridging structural understanding with therapeutic strategy

TrkB (neuronal receptor tyrosine kinase-2, NTRK2) is the receptor for brain-derived neurotrophic factor (BDNF) and is a critical regulator of activity-dependent neuronal plasticity. The past few years have witnessed an increasing understanding of the structure and function of TrkB, including its transmembrane domain (TMD). TrkB interacts with membrane cholesterol, which bidirectionally regulates TrkB signaling. Additionally, TrkB has recently been recognized as a binding target of antidepressant drugs. A variety of different antidepressants, including typical and rapid-acting antidepressants, as well as psychedelic compounds, act as allosteric potentiators of BDNF signaling through TrkB. This suggests that TrkB is the common target of different antidepressant compounds. Although more research is needed, current knowledge suggests that TrkB is a promising target for further drug development.

Targeting the postsynaptic scaffolding protein PSD-95 enhances BDNF signaling to mitigate depression-like behaviors in mice

Signaling mediated by brain-derived neurotrophic factor (BDNF), which is supported by the postsynaptic scaffolding protein PSD-95, has antidepressant effects. Conversely, clinical depression is associated with reduced BDNF signaling. We found that peptidomimetic compounds that bind to PSD-95 promoted signaling by the BDNF receptor TrkB in the hippocampus and reduced depression-like behaviors in mice. The compounds CN2097 and Syn3 both bind to the PDZ3 domain of PSD-95, and Syn3 also binds to an α-helical region of the protein. Syn3 reduced depression-like behaviors in two mouse models of stress-induced depression; CN2097 had similar but less potent effects. In hippocampal neurons, application of Syn3 enhanced the formation of TrkB-Gαi1/3-PSD-95 complexes and potentiated downstream PI3K-Akt-mTOR signaling. In mice subjected to chronic mild stress (CMS), systemic administration of Syn3 reversed the CMS-induced, depression-associated changes in PI3K-Akt-mTOR signaling, dendrite complexity, spine density, and autophagy in the hippocampus and reduced depression-like behaviors. Knocking out Gαi1/3 in hippocampal neurons prevented the therapeutic effects of Syn3, indicating dependence of these effects on the TrkB pathway. The findings suggest that compounds that induce the formation of PSD-95-TrkB complexes have therapeutic potential to alleviate depression.